Probably Aipom or Ambipom. Both of these Pokemon have hands for tails, so I'd imagine it'd be very easy for them to take a selfie. They seem to have self awareness, so, a natural pick.
Gotta agree with this, I really think Aipom would take some great selfies, even some mischievous ones whilst stealing some food or something haha. I suppose the first one that comes to mind is the other mascot Pokemon like Pikachu who seem pretty self-aware as well, it's boring, but I imagine Pikachu could take some really cute selfies :)